This invention, broadly, relates to the repair of crash-resistant fuel tanks for helicopters and light aircraft.
The United States Army is deeply committed to improving the safety and survivability of its helicopter fleet. Two major developments have provided significant advances in this area, the ballistic and crashworthy fuel systems, and the crash attenuated armored pilot seats. The former provides protection from post-crash fires and the latter affords protection from crash impact forces.
The need for crash-resistant fuel systems to eliminate or minimize the probability of post-crash fires was demonstrated in Southeast Asia where crewmen died from such fires in otherwise survivable accidents. The need for ballistic tolorances also became evident at that time.
The flammable helicopter fluids are fuel, oil, and hydraulic fluid. The confinement of these fluids in a crash prevents the formation of readily combustible mixtures and, in addition, isolates the flammable fluids. The major crash-fire hazards, therefore, generally are associated with the fuel. It has been concluded that efforts toward a solution of the crash-fire problem can be concentrated mainly on the fuel tanks and those components and accessories which are associated directly with the tanks.
In the development of crash-resistant fuel systems the approach has been to build an elastomeric fuel cell capable of withstanding a scientifically determined maximum survivable crash impact at a velocity of 64.7 feet per second. Its derivation is the result of extensive analysis of accident data and corresponds to the 95 percentile accident of a fixed wing transport aircraft. However, it long has been recognized that the ability of a fuel cell to remain intact during a crash is influenced to an appreciable extent by the accessories which are attached to it. These accessories include filler necks, fuel pumps, vents, interconnectors, outlets, fuel quantity gauges, drains, hanger fittings, fuel dump valves, and so forth. Because of rigid attachment to the aircraft structure, many of these accessories have been found in actual crashes to tear or otherwise damage the cells. During crashes, forces often exist which tend to move the fuel cell with respect to adjacent cells or with respect to the aircraft structure which surrounds and supports it. This movement often causes fuel-cell tearing at points of attachment. This means that most of the tears occur at the interface of metal and elastomeric material. Such tears pose a particularly difficult repair problem. Many materials simply will not form sufficiently strong bonds to both metal and elastomers. An equally difficult problem is that of repairing ballistically punctured crashworthy fuel cells. Amplifying the problem has been the requirement that cells be repaired in twenty-four hours at ambient temperatures. The composition of the fuel tank was also a factor in the solution of the problem.
In an effort to solve these problems extensive research was conducted. Since the fuel tanks are generally rubber-impregnated nylon it was found that if the bonding agent was too rigid stresses were created causing cell rupture. Many bonding agents did not have sufficient adhesion to be suitable. Others lacked resistance properties required of a fuel cell.
Rendering many bonding agents unsuitable was the variety of substrates. It was necessary to obtain excellent adhesion to metal, to nylon, and to various plastics forming or sealing the cell. A composition forming a strong bond with one of the materials would not necessarily bond the other materials. Still other bonding matrixes did not meet the twenty-four hour curing schedule. It is indeed difficult to find a single bonding matrix which is totally satisfactory.